∑\sum∑ 这个求和符号的优先级和 +++ 号那个大？ ∑i=1nxi2pi−[E(X)]2\sum_{i=1}^n x^{2}_{i}p_{i}-[E(X)]^2∑i=1nxi2pi−[E(X)]2 是看成∑i=1n(xi2pi−[E(X)]2)\sum_{i=1}^n (x^{2}_{i}p_{i}-[E(X)]^2)∑i=1n(xi2pi−[E(X)]2) 还是 (∑i=1nxi2pi)−[E(X)]2(\sum_{i=1}^n x^{2}_{i}p_{i})-[E(X)]^2(∑i=1nxi2pi)−[E(X)]2
